THS7313 datasheet(22/48 Pages) TI

22 / 48 page

www.ti.com

INPUT MODES OF OPERATION – AC SYNC TIP CLAMP

Input

Internal

Circuitry

STC

LPF

STC

Bias

Select

1.8 A

5.8 A

7.8 A

135 mV

Comparator

0.1 F

VS+

THS7313

SLOS483 – NOVEMBER 2005

APPLICATION INFORMATION (continued)

The last input mode of operation is the ac with sync-tip-clamp (STC) which also requires a capacitor in series

with the input. Note that while the term sync-tip-clamp is used throughout this document, the THS7313 is better

termed as a dc restoration circuit based on the way this function is performed. This circuit is an active clamp

circuit and not a passive diode clamp function. This function should be used when ac coupling is desired with

signals that have sync signals embedded such as CVBS, Y’, and G’ signals.

The input to the THS7313 has an internal control loop which sets the lowest input applied voltage to clamp at

approximately 135 mV. Like the dc + 135 mV input shift, the resulting output voltage low level is about 270 mV. If

the input signal tries to go below the 135-mV level, the internal control loop of the THS7313 sources up to 2 mA

of current to increase the input voltage level on the THS7313 input side of the coupling capacitor. As soon as the

voltage goes above the 135-mV level, the loop stops sourcing current.

One of the concerns about the sync-tip-clamp level is how the clamp reacts to a sync edge that has overshoot –

common in VCR signals or reflections found in poor PCB layouts. Ideally the STC should not react to the

overshoot voltage of the input signal. Otherwise, this could result in clipping on the rest of the video signal

because there may be too much increase in the bias voltage.

To help minimize this input signal overshoot problem, the patent-pending internal STC control loop in the

THS7313 has an I2C selectable low-pass filter as shown in Figure 51. This filter can be selected to be about

500 kHz, 2.5 MHz, or 5 MHz. The effect of this filter is to slow down the response of the control loop so as not to

clamp on the input overshoot voltage, but rather the flat portion of the sync signal. If the input signal is known to

be well controlled – such as coming from a video encoder – then selecting the 5-MHz filter should yield excellent

results. But, if the input signal is coming from an unknown source, then selecting the 2.5-MHz or 500-kHz filter

may yield the best results. There is a noticeable delay of the STC loop when the 500-kHz filter is selected.

As a result of this selectable delay, the sync has an apparent voltage shift occurring between 15 ns and 2

µs –

depending on the STC LPF. The amount of shift is dependant upon the amount of droop in the signal as dictated

by the input capacitor and the STC input bias current selection. Because the sync is primarily for timing purposes

with syncing occurring on the edge of the sync signal, this shift is transparent in most systems. Note that if the

source signal is known to be good, selecting the 5-MHz STC LPF is recommended for all sources

While this feature may not fully eliminate overshoot issues on the input signal in case of bad overshoot and/or

ringing, the STC system helps minimize improper clamping levels. As an additional method to help minimize this

issue, an external capacitor (example: 10 pF to 47 pF) to ground in parallel with the external termination resistors

can help filter overshoot problems.

It should be noted that this STC system is dynamic and does not rely upon timing in any way. It only depends on

the voltage appearing at the input pin at any given point in time. The STC filtering helps minimize level shift

problems associated with switching noises or short spikes on the signal line. This helps ensure a robust STC

system.

Figure 51. Equivalent AC Sync Tip Clamp Input Mode Circuit

THS7313 Datasheet(PDF) 22 Page - Texas Instruments

THS7313 Datasheet(HTML) 22 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL